1. Field of the Invention
This invention relates generally to polarizing beam splitters, and more particularly, to broad band polarizing beam splitters.
2. Description of Related Art
Beam splitter cubes consist generally of matched pairs of identical right angled prisms, with their hypotenuses faces cemented together. Prior to cementing, a partial reflection film is deposited onto one of the hypotenuse faces. A beam splitter cube can be made conveniently by using a thin, low-index transparent film as a precision spacer. Low-loss reflectors, whose transmittance can be controlled by frustrating internal reflection, are of considerable practical interest.
Cube beam splitters have several advantages over plate beam splitters and are widely used for a variety of reasons. Cube beam splitters deform much less when subjected to mechanical stress than a plate beam splitter. Traditionally, when cube beam splitters are used in convergent or divergent portions of an optical beam, they contribute substantial amounts of unwanted aberration. This is avoided or minimized by only using cube beam splitters with collimated, or nearly collimated beams.
Polarizing beam splitters are known in the art. These beam splitters typically have two prisms that are cemented together, hypotenuse-face-to-hypotenuse-face, with a special multilayer dielectric film in between. Monochromatic unpolarized light which is normally incident upon the external faces of the resulting cube (internally incident at 45 degrees upon the multi-layer film) is separated into two polarized beams which emerge from the cube through adjacent faces and in directions which are accurately 90 degrees apart. The beam, which passes straight through the cube, emerges linearly polarized with the plane of the electric field vector parallel to the plane of incidence defined for the multilayer film (p-polarized). The beam which emerges from the cube at right angles to the incident beam, having been reflected by the multi-layer film, is linearly polarized with the electric field vector orthogonal to the plane of incidence defined for the multilayer film (s-polarized).
At every dielectric interface within the stack of the multilayer film, the radiation is incident at or near Brewster's angle, and the p-polarized component is therefore transmitted with almost no reflection. These polarizing cubes are suitable for most of the monochromatic applications to which other polarizers might be applied, such as isolator construction, or the construction of electro-optic or magneto-optic modulators. They can also be used in laser Q-switches, though the use of an optical cement and the multilayer film prohibits use at high power. However, polarizing cubes have limitations. Light must be incident on the beam splitter coating at an angle of 45 degrees plus or minus 2 degrees. The angular sensitivity of these beam splitters not only requires that the cube be positioned accurately, but that only collimated or nearly collimatedbeams of light be used.
MacNeille polarizers are based on the principle that it is always possible to find an angle of incidence so that the Brewster condition for an interface between two materials of differing refractive index is satisfied. When this is so, the reflectance for the p-plane of polarisation vanishes. The s-polarized light is partially reflected and transmitted. To increase the s-reflectance, retaining the p-transmittance at or very near unity, the two materials may then be made into a multilayer stack. The layer thicknesses should be quarter-wave optical thicknesses at the appropriate angle of incidence. The construction of MacNeille beam splitters uses two prisms with a multi-layer dielectric between two faces of opposing right angled prisms. The advantage of a MacNeille polarizing beam splitters over other polarizers, such as the pile-of-plates polarizer, is its wide spectral range coupled with a large physical aperture. However, MacNeille beam splitters suffer from limited angular field, particularly at the center of its range, simply because the Brewster condition is met exactly only at the design angle.
Edge filters have also served as polarizing beam splitters. They also suffer the disadvantage of not covering the entire visible spectrum. When edge filters are used in color projection systems, one edge filter is used for the red, another one for the green, and a third one for the blue.
It would be desirable to provide to polarizing beam splitter with broad angular acceptance and a broad band width. It would be further desirable to provide a polarizing beam splitter with good throughput and that is less sensitive to errors in, (i) layer thicknesses, (ii) film deposition, and (iii) the refractive index of the layers.